REACTIVE MEMBRANE FOR FILTRATION AND PURIFICATION OF GASES OF IMPURITIES AND METHOD UTILIZING THE SAME

Abstract: A reactive membrane for removing impurities, such as water, oxygen, organic compounds, and metallic compounds from a gas is provided. The reactive membrane includes a porous substrate and at least one carbon layer, which is modified to present active sites, deposited on the porous substrate. The active sites include metal species which are at least partially deoxygenated and are chemically bonded to the carbon layer. Methods of forming the reactive membrane, of activating and regenerating the active sites, and of removing impurities from a gas with the membrane are also provided.

Description: REACTIVE MEMBRANE FOR FILTRATION AND PURIFICATION OF GASES OF IMPURITIES AND METHOD UTILIZING THE SAME Field of the Invention This invention relates to a reactive membrane, a method of forming the reactive membrane, and a method of filtering and purifying gases of impurities. Background of the Invention In order to manufacture advanced materials such as semiconductor devices, trace impurities on the parts per million (ppm) to parts per billion level (ppb) and even sub-ppb levels in any of the manufacturing constituents must be substantially avoided. For example, in manufacturing semiconductor devices, impurities (e.g., moisture, oxygen and organic compounds) in process gases adsorb on the semiconductor wafer, degrade its performance, and lower the manufacturing yield and device reliability. There are generally two types of impurities in process gases: heterogeneous (e.g., particles and acromolecules) and homogeneous (e.g., dissolved particles) . Typically, heterogeneous impurities are removed by filtration and homogeneous impurities are removed by purification. There has been particular interest in the removal of homogeneous impurities from process gases. U.S. Patent No. 5,196,380 to Shadman discloses a reactive membrane for the removal of homogeneous impurities from a gas stream. The reactive membrane comprises a ceramic substrate, a carbon layer coating the substrate, and a reactive layer including metal species in reduced form bound to the carbon layer. Although this membrane is effective in the removal of homogeneous impurities, it has several limitations. The affinity or bond between the carbon layer and the ceramic substrate is very important. If the affinity or bond is not strong enough, the carbon may disassociate from the ceramic substrate and contaminate the gas stream during filtration/purification. The carbon layer coating the ceramic substrate may also exhibit a lack of uniformity. Additionally, according to U.S. Patent No. 5,196,380, the reactive membrane is formed by impregnating the carbon layer with the metal species using a solution of metal-bearing compounds. The carbon layer and metal species are then heated in an inert atmosphere to activate the reactive membrane. Unfortunately, residual solvent, namely water, remaining on the carbon layer after impregnation with metal species may interfere with the subseguent activation of the reactive layer. This is particularly troublesome if the residual solvent is potentially oxidizing or strongly chemisorbed on the metal species. Further, in an inert atmosphere, the activation and regeneration of the metal species bound to the carbon layer require extended periods of time and high temperatures in order to effect reduction of the metal species and formation of the reactive layer. Summary of the Invention The present invention provides a reactive membrane for removing impurities from a gas comprising a porous metal substrate and at least one carbon layer deposited on the porous metal substrate, the carbon layer being modified to present active sites which include at least partially deoxygenated metal species chemically bonded to the carbon layer. The present invention also provides a method for forming a reactive membrane for removing homogeneous and heterogeneous impurities from gases, said method comprising depositing at least one layer of carbon on a porous metal substrate, depositing a precursor metal species on the carbon layer, and heating the porous metal substrate to form active sites on the carbon layer, wherein the active sites include at least partially deoxygenated metal species chemically bonded to the carbon layer. A reactive membrane embodying the present invention and including a porous metal substrate provides many advantages over conventional reactive membranes. For example, the affinity or bond between the carbon layer and the porous metal substrate is greater than the affinity or bond between the carbon layer and a porous ceramic substrate. Thus, the carbon may be more readily and more efficiently deposited on the porous metal substrate. Further, during filtration/purification, the stronger affinity or bond between the carbon layer and the porous metal substrate reduces the possibility of contamination of the gas stream and clogging of the pores of the membrane with carbon particulates that might otherwise sluff off the substrate. The carbon layer also coats the porous metal substrate more uniformly than the ceramic substrate, providing a more uniform surface for subsequent impregnation with the precursor metal species, thus forming an improved reactive membrane. Another advantage is that metal substrates can be made in many different shapes and forms, and with a wide variety of pore sizes. This provides a wider choice of membrane geometries as well as better control of filtration parameters, such as pore rating and pressure drop, through pore size selection. Additionally, the membrane will frequently be placed in a metal housing, often requiring the membrane to be attached to the housing. A metal substrate is more easily and reliably welded to a metal housing, creating a better and stronger seal. Welding a dissimilar material, such as a ceramic substrate, to a metal housing is more difficult particularly under temperature excursions and could result in a weak seal, allowing the gas stream to bypass the membrane. The improved seal between the metal housing and the metal substrate would virtually eliminate the possibility of bypass. The present invention further provides a method for forming a reactive membrane for removing homogeneous and heterogeneous impurities from gases, said method comprising depositing at least one layer of carbon on a porous substrate, depositing a precursor metal species on the carbon layer by contacting the carbon layer with a precursor metal species in a non-aqueous solvent, and heating the carbon layer to form active sites including at least partially deoxygenated metal species chemically bonded to the carbon layer. A method embodying the invention and using a non-aqueous solvent to deposit the metal species on the carbon layer has many advantages. For example, after the mixture of the non-aqueous solvent and the metal species contact the carbon layer, the non- aqueous solvent may be easily evaporated from the carbon layer without leaving any residual solvents. This is particularly important if the residual solvent is potentially oxidizing or is strongly chemisorbed to the metal species. Either would interfere with and hinder the subsequent thermal activation step to form the reactive layer. In a preferred embodiment, anhydrous liquid ammonia is used as the non-aqueous solvent. Not only does anhydrous liquid ammonia evaporate easily, but it is inherently reducing, not oxidizing, and therefore facilitates the activation of the metal species as the carbon layer is heated to form active sites. Additionally, since the metal sites have never been exposed to water when anhydrous liquid ammonia is used as the solvent, the difficulties in removing strongly chemisorbed water molecules from the metal sites are avoided entirely. Finally, anhydrous liquid ammonia readily dissolves many of the metal species, providing high loadings and uniform distribution of the metal species on the carbon layer. Additionally, the present invention provides a method for activating a reactive membrane comprising heating a porous substrate which includes a carbon layer having active sites to greater than about 300°C in a reducing atmosphere. The present invention further provides a method for processing a gas stream comprising establishing a flow of gas through a reactive membrane comprising a porous inorganic substrate and at least one carbon layer deposited on the porous substrate, the carbon layer being modified to present active sites which include metal species; interrupting the gas flow through the reactive membrane; heating the reactive membrane to greater than about 300°C in a reducing atmosphere, thereby regenerating the active sites; and reestablishing the gas flow through the reactive membrane. A method embodying the invention and using a reducing atmosphere for activation and/or regeneration of the metal species is particularly advantageous. For example, a reducing atmosphere facilitates active site formation by reducing the time and temperature necessary to partially or substantially deoxygenate the precursor metal species and bind the metal species to the carbon layer. Further, during regeneration of the active sites in a reducing atmosphere containing hydrogen, reduction and deoxygenation of the metal species may utilize the hydrogen gas instead of the carbon layer as the reducing agent. This diminishes the consumption of the carbon layer during regeneration and may prolong the life of the reactive membrane. Description of a Preferred Embodiment The present invention will now be described more fully hereinafter. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein; rather, this embodiment is provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The present invention provides a reactive membrane for filtering and purifying gases of impurities. The reactive membrane includes a porous inorganic substrate, preferably a porous metal substrate, and at least one layer of carbon deposited on the porous inorganic substrate. The carbon layer is modified to present active sites, which include at least partially deoxygenated metal species (metal in reduced form) chemically bonded to the carbon layer. The metal in reduced form is capable of interacting and, preferably, reacting with the impurities, thereby scavenging and removing the impurities from the gas. The substrate layer has a plurality of pores and is effective in removing heterogeneous impurities (i.e., particles and macromolecules) from the gas being filtered and purified. The porous substrate is resistant to temperatures greater than about 300°C, and preferably temperatures greater than about 500°C, to permit the reactive membrane to be regenerated after use, i.e., after deactivation by interaction with impurities. The porous inorganic substrate may include a porous ceramic material or other porous inorganic material, such as porous carbon. Porous substrates which include a porous ceramic material preferably have an initial pore size of about 50θA. Any of the commercially available ceramic materials having a porous structure and exposed surfaces (e.g., alumina, zirconia, titania) are suitable for use in the subject invention as the porous substrate. An exemplary porous ceramic membrane is sold under the trademark Membralox® by Alcoa of Warrendale, Pennsylvania. Membralox® is an α-alumina membrane coated with a monolayer of 7-alumina and is available in a variety of pore sizes. Exemplary configurations of the porous substrate may include a bundle of ceramic tubes in a shell or a wall flow monolithic geometry. A preferred material for the porous substrate is a porous metal. Porous substrates which include a porous metal typically have a larger initial pore size, e.g., on the order of 2 to 5 microns, than a porous ceramic substrate. Many commercially available porous metal materials may be used, but porous metal filters formed, for example, by sintering a metal powder or metal fibers are particularly preferred. An exemplary porous stainless steel substrate is a porous 316L stainless steel substrate which is formed from sintered stainless steel fibers and has an initial pore size of about 4 microns. Such stainless steel substrates are commercially available from Pall Corporation of East Hills, New York, USA. Many of the advantages of a porous metal substrate over a porous ceramic substrate relate to the deposition of the carbon layer on the substrate. When the carbon layer is deposited on the porous metal substrate, it is deposited as a carbon film. This carbon film has a stronger bond or affinity for porous metal substrates in comparison to ceramic substrates which provides for less loss of carbon particulates during filtration/purification, reducing the possibility of contamination of the gas stream and fouling of the pores of the substrate. Further, the carbon layer can be more efficiently and uniformly deposited on a porous metal substrate. The carbon layer is preferably deposited on substantially all of the exposed surfaces of the porous inorganic substrate. The thickness of the carbon layer deposited on porous substrates which include a porous ceramic material is preferably from about 2θA to about lOoA. Porous substrates which include a porous metal substrate typically have thicker carbon layers deposited on their exposed surfaces. The deposited layer of carbon is in the form of a carbon film, which has a strong affinity or bond to the exposed surfaces (i.e., the carbon is not deposited as fluffy carbon or carbon filaments) , to prevent carbon particulates from sloughing off the substrate surfaces and possibly clogging the pores of the membrane. If a porous ceramic substrate is used, the ceramic material preferably includes a metallic impurity, e.g., about 0.1 to about 1% of iron or nickel, to facilitate deposition of the carbon. Typically, the porous substrate is degassed prior to the deposition of the carbon layer to remove any preexisting impurities in the substrate. The carbon layer preferably is deposited over and coats substantially the entire surface area of the porous substrate (i.e., substantially all of the exposed surfaces of the porous substrate) and reduces the pore size of the substrate. For example, deposition of the carbon layer may reduce the pore size of a porous ceramic material from about 50θA to a value in the range from about 5θA to about lOOA. An exemplary technique for depositing the carbon layer is a CVD technique via the disproportionation of carbon monoxide (CO) or the dissociation of other carbon sources such as various hydrocarbons. The deposition can occur under various reaction times and temperatures, and gas compositions, the control of which being within the skill of one in the art. For example, if CO is used as the carbon source, the disproportionation may be conducted at a temperature above about 250°C using a gas mixture of about 5 to 15 percent by volume CO, 1 to 5 percent by volume hydrogen and the remainder a relatively inert gas such as nitrogen. Preferably, the disproportionation is carried out using a mixture of CO and hydrogen, which contains from about 6% to about 14% hydrogen (by volume) , at a temperature of from about 350°C to about 450°C. The thickness and amount of carbon deposited may be controlled by adjusting the reaction time, temperature, gas composition and gas flow rate. Carbon deposition is typically more uniform and efficient with metal substrates or with ceramic substrates which include metallic impurities. For example, good carbon deposition may be achieved at about 435°C on a 1.1" diameter filter disk formed from sintered stainless steel fibers using an about 88/12 (vol/vol) mixture of CO and hydrogen at a total gas flow rate through the filter disk of about 500 cc/min. The active sites include at least one metal species in reduced form chemically bonded to the carbon layer. The metal species is capable of reacting or otherwise interacting with the impurities of the gas stream. The term "in reduced form" relates to the oxidation state of the metal in that the oxygen is present in less than the stoichiometric amount in the metal oxide (i.e., the metal species is partially, substantially, or completely deoxygenated) . The reduced form of the metal may include metal or metal suboxides chemisorbed on carbon as well as metal-carbon intercalate compounds. In other words, the carbon layer is modified to present active sites which include partially, substantially, or completely deoxygenated metal species chemically bonded to the carbon layer. Preferably, the active sites include substantially deoxygenated metal species which are capable of reacting with trace impurities in a gas stream. Exemplary metals include manganese, magnesium, and alkali metals such as lithium, sodium and potassium. The formation of the active sites may be carried out by first depositing a precursor metal species on the carbon layer. The precursor metal species may include an oxidized form of the metal and/or a deoxygenated form of the metal. The precursor metal species may be deposited by using a CVD technique, e.g., forming a vapor of the precursor metal species to be deposited and contacting the precursor metal species vapor with the layer of carbon deposited on the porous substrate layer. The vapor, for example, may be generated from the metal itself or from a hydroxide or oxide of the metal. The temperature for deposition depends on the specific porous substrate and precursor metal species and, typically, is greater than about 300°C. Alternately, the precursor metal species may be deposited from a mixture which includes a solvent and the precursor metal species. The mixture of the precursor metal species and the solvent may be either a solution or a slurry. In a solution, the precursor metal species dissolve in the solvent, forming a homogeneous mixture. In a slurry, the precursor metal species are suspended in the solvent, forming a finely dispersed heterogeneous mixture. Solutions of metal-bearing compounds such as metal hydroxides or oxalates may be employed to impregnate the carbon-coated substrate with the metal precursor species. For example, if manganese is used, an aqueous solution of manganese oxalate is suitable for this purpose. A preferred method of depositing the precursor metal species is to dissolve or slurry the precursor metal species in a non-aqueous solvent. Preferably, the non-aqueous solvent is a solvent which may be easily evaporated without leaving a residue, e.g., a solvent such as anhydrous liquid ammonia. Use of a non-aqueous solvent which may be easily evaporated is a distinct advantage of the present invention in terms of ease of preparation and uniformity of deposition. Further, the absence of water or other oxidizing solutions during the precursor metal deposition step allows less severe conditions, such as less time or lower temperature, to be employed in the subsequent activation of the metal species where the metal species are partially, substantially, or completely deoxygenated. Exemplary mixtures of a precursor metal species in a non-aqueous solvent include solutions of an alkali metal, such as sodium, in anhydrous liquid ammonia. If metals, which are insoluble or only sparingly soluble in a non-aqueous solvent such as anhydrous liquid ammonia, are to be used as the precursor metal species, a slurry of a very fine powder of the metal and the non-aqueous solvent may be suitable for depositing the metal on the carbon layer. The mixture of the precursor metal species is passed through the carbon-coated porous substrate, thereby depositing the precursor metal species on the carbon layer. Nitrogen or another inert gas may be used to force the mixture through the porous substrate or suction may be used to draw the mixture through the porous substrate. After passing the mixture through the porous substrate to deposit the metal precursor species on the carbon layer, the non-aqueous solvent may be driven off by purging with an inert gas such as nitrogen. Gradual heating to about 110°C and, preferably 200°C or greater, may facilitate the purging process. The substrate bearing the deposited precursor metal species is then heated to form metal species chemically bonded to the carbon layer, i.e., to form active sites on the carbon layer. The term "chemically bonded" is intended to include ionic and covalent bonds and van der Waals forces. In other words, "chemically bonded" includes absorbed metal and metal suboxides on carbon as well as various metal-carbon groups and intercalate compounds. Typically, the chemical bonding is accomplished by heating the substrate to greater than about 300°C, preferably about 380°C to about 400°C, and most preferably about 500°C, thereby forming the active sites on the carbon layer. The formation of the active sites may be carried out by heating the substrate in an inert atmosphere such as nitrogen, argon, helium, and the like. If the active sites include lithium, potassium or magnesium species, argon or helium is preferably used to provide the inert atmosphere. The presence of the inert atmosphere lessens the opportunity for impurities and, in particular, oxidizing impurities, in the atmosphere to come into contact with the membrane being activated. If impurities are present in the gas in contact with the membrane during the activation process, the impurities may interact with and consume active sites. The loss of active sites may adversely affect the impurity removal capabilities of the reactive membrane. Preferably, the activation process is carried out under ultra pure inert gas to avoid the deactivation of active sites due to trace impurities in " the inert gas. The inert gas may also include a reducing gas, e.g., hydrogen, to facilitate active site formation. Use of a reducing gas such as hydrogen during the activation process may significantly reduce the time and/or temperature required to form the active sites. The presence of the reducing gas promotes the partial, and preferably substantial, deoxygenation of the metal species, forming the desired reduced form of the metal as active sites. The presence of the reducing gas also reduces the consumption of the carbon layer as the reducing agent during regeneration, thus prolonging the life of the reactive membrane. Preferably, the inert gas employed during the activation process includes at least about 1 percent, more preferably from about 2 to about 35 percent, most preferably from about 4 to about 10 percent hydrogen. In another embodiment, the activation process may be carried out by heating the substrate to greater than about 300°C under vacuum. Typically, the substrate is heated to greater than about 500°C and maintained at that temperature for a suitable amount of time to form the active sites (e.g. , about one hour) while being purged with a mixture of an inert gas preferably having at least about 4 percent hydrogen. This activates the active sites of the carbon layer by reacting the precursor metal species with the carbon to form active sites on the substrate, i.e., to form metal species chemically bonded to the carbon layer. The metal active sites may exist in the reduced form (C-M) , as an adsorbed metal, as a metal suboxide, or in oxidized form (C0 2 M) , with the removal of the oxygen necessary to have the preferred highly reactive reduced form. Stated otherwise, the metal in reduced form has no, or less than a stoichiometric amount of oxygen, i.e., the metal species is partially, substantially, or completely deoxygenated. Preferably, the metal species is substantially deoxygenated. The chemical bond anchors the reactive metal species to the carbon and the carbon is anchored to the porous substrate, especially a porous metal substrate. Thus, because the metal species and carbon are anchored, contamination of the gas stream during filtration/purification is avoided. In another embodiment, several different metals can be deposited sequentially or as a mixture onto the carbon layer to permit removal of selective impurities as the gas passes through each of the layers of the filter membrane. In still another embodiment, the reactive membrane may include two or more layers of carbon deposited on the exposed surfaces. Each carbon layer may be individually modified to present active sites. Such membranes may be produced by sequentially depositing a carbon layer, depositing a precursor metal species on the carbon layer and then heating the membrane to form the active sites on the carbon layer. This sequence may be repeated until the desired number of activated carbon layers has been produced. Reactive membranes, which include at least two carbon layers with each layer presenting active sites, have a number of advantages. Such multi-carbon layer membranes have a very high impurity removal capacity. The active sites on the carbon layers may include more than one metal species. For example, a reactive membrane may include first and second carbon layers deposited on substantially all of the exposed surfaces of the porous inorganic substrate. The first carbon layer may be modified to present active sites which include at least partially deoxygenated first metal species chemically bonded to the first carbon layer and the second carbon layer may be modified to present active sites which include at least partially deoxygenated second metal species chemically bonded to the second carbon layer. This may permit the use of two metals which require different deposition methods (e.g., one via CVD and the other via non-aqueous solution deposition) . The present reactive membranes are highly effective for removing a variety of impurities from a gas. For example, a reactive membrane embodying the present invention may be used to remove an impurity, such as oxygen (0 2 ) , water (H 2 0) , carbon monoxide (CO) , carbon dioxide (C0 2 ) or methane (CH 4 ) , from a gas stream, e.g., a stream of an inert gas such as nitrogen or argon. By passing the gas including the impurities through the reactive membrane and interacting the impurities with the active sites, the concentration of the impurity in the gas may be reduced to no more than about 100 ppb, preferably to no more than about 1 ppb. By employing a reactive membrane embodying the present invention, the concentration of water or carbon monoxide in an inert gas may be reduced to no more than about 0.7 ppb. A reactive membrane embodying the present invention may be employed to reduce the concentration of methane in an inert gas to no more than about 0.2 ppb. A reactive membrane embodying the present invention may also be employed to reduce the concentration of oxygen or carbon dioxide in an inert gas to no more than about 0.1 ppb. The reactive membranes of this invention may find use in a variety of applications. For example, the reactive membranes may be employed in the removal of trace gas contaminants such as 0 2 , C0 2 , H 2 0, CO and CH 4 at the point of use in electronic manufacturing processes. They also may find use in the removal of gaseous and particulate contaminants in gas or liquid source streams used in electronics manufacture, e.g., the removal of metal oxides that may be formed in tubing. Additionally, the reactive membranes embodying this invention may find use in the treatment of exhaust gas streams from various processes, e.g., in the removal of small amounts of cyanides, silanes, arsenic, mercury and the like, including poisonous particulates which may be present in gaseous exhaust streams from processes used in electronics manufacturing. The reactive membranes are also extremely effective for the purification of inert gases, even at temperatures as low as room temperature. For example, the reactive membranes of the present invention may be used to remove impurities, such as oxygen, moisture, carbon dioxide and hydrocarbons, from inert gases. The present invention permits the achievement of levels of purification that are significantly better than those which may be realized with the best commercial purification systems. The present reactive membranes may also be used to remove moisture from streams of reactive gases such as oxygen, silanes, hydrogen chloride (HCl) and hydrogen bromide (HBr) . Typically, reactive membranes having active sites which include sodium or magnesium species chemically bonded to the carbon layer may be employed for this purpose. Even though the reactive gas may react with the active sites, moisture may still be chemisorbed by the reactive membrane (i.e., the water interacts with the active sites, thereby substantially removing the water from the reactive gas) . For example, although HCl or HBr may react with an active site (e.g., active sites which include magnesium species) to form a metal halide, moisture may still be chemisorbed by the metal halide to form a hydrate. Similarly, oxygen may react with the metal species of the active sites to form an oxidized site. Since the oxidized sites are capable of interacting with moisture, the reactive membranes of the present invention may be employed to remove trace levels of moisture from a reactive gas like oxygen. Reactive membranes embodying the present invention may also be used to remove oxygen from silanes. In operation, a gas stream including impurities is passed through the reactive membrane, which may have any one of a variety of known configurations. It is recognized that a liquid stream could also be filtered/purified by altering the membrane in a manner known to those skilled in the art. Heterogeneous particles are removed from the gas stream by well known filter mechanisms. The active sites of the membrane interact, and preferably react, with trace homogeneous impurities present in the gas stream (e.g. oxygen, metallic compounds and the like, moisture or organic compounds) thereby removing the homogeneous impurities from the gas stream. Although applicant does not wish to be bound by any theory, it is thought that substantially all of the impurity molecules come in contact with the metal molecules of the reactive layer, interact, and are removed by the membrane from the gas being filtered and purified. In other words, the impurities interact with the metal species which are bonded to the active sites of the carbon layer, thereby scavenging and removing the impurities from the gas phase. Once purification is complete and all of the reduced metal is oxidized or otherwise deactivated, the membrane may be regenerated by heating the substrate to greater than about 300°C in an inert atmosphere (e.g., nitrogen or argon) to reduce the oxidized metal (i.e., regenerate the active sites). Alternatively, the regeneration may be carried out by heating the deactivated reactive membrane to greater than about 300°C under a reducing atmosphere, such as a hydrogen-containing atmosphere. The regeneration capability of the reactive membrane, a process which may be carried out many times, provides a significant advantage over known filtration/purification materials which typically may be used only once and for which there are apparently no known means of regeneration. The membrane may be regenerated after use by heating the membrane to at least about 300°C, preferably to greater than about 450°C, most preferably to a temperature of about 450°C to about 550°C, in an inert atmosphere (e.g., nitrogen, argon, helium and the like) . The carbon layer may participate in regeneration of the active sites in that, on heating in an inert atmosphere, the carbon may function as a reducing agent. The regeneration process is typically carried out for a time period of about 24 to 28 hours, although longer time periods, e.g., about 48 hours, may also be employed. Alternatively, the reactive membrane may be regenerated after use by heating the membrane to greater than about 300°C, and preferably to a temperature of about 450°C to about 550°C, in the presence of a reducing atmosphere, such as a hydrogen-containing atmosphere. For example, the membrane may be regenerated by heating to greater than about 500°C and maintaining the membrane at that temperature for about one hour or more while it is purged with a mixture of hydrogen and an inert gas. Preferably, the regeneration process is carried out using an atmosphere of an inert gas having from about 2 percent to about 100 percent hydrogen (vol:vol) , more preferably from about 10 percent to about 40 percent hydrogen. Typically, the regeneration of the reactive membrane is carried out for a time period of from about 8 to about 12 hours under these conditions. As with the activation process used to originally form the active sites, the regeneration may also be carried out under an inert gas containing a lesser amount of hydrogen. With lower concentrations of hydrogen, longer times and/or higher temperatures are typically employed for the regeneration process. For example, the regeneration of a stainless steel reactive membrane may be carried out by heating the membrane for 12 to 24 hours at about 500°C while purging with a 35:65 (vol:vol) mixture of hydrogen and nitrogen. The filtration of heterogeneous impurities and the purification of homogeneous impurities may occur substantially simultaneously, i.e., with the heterogeneous impurities being trapped by the pores of the substrate and the homogeneous impurities interacting with the active sites of the carbon layer deposited on the substrate. Alternatively, the two different types of impurities may be removed by different regions of the reactive membrane. For example, the reactive membrane may include a separate porous region, e.g., an upstream layer of porous medium, which functions to substantially filter the heterogeneous impurities from the gas. In another embodiment of the invention, a separate porous medium, (e.g., a ceramic filter) which is placed upstream of the reactive membrane may be used to remove substantially all of the heterogeneous impurities from the gas stream. The present invention is further illustrated by the following examples. Example 1 Sodium Species on Carbon-Coated Porous Stainless Steel A porous stainless steel disk (1.1" diameter, 0.25" thick; formed by sintering about 2 micron 316L stainless steel fibers) having a pore size of about 4 microns is degassed under a nitrogen atmosphere at about 400°C to remove any impurities. A carbon film layer is deposited on substantially all of the exposed surfaces of the substrate using the following procedure. The stainless steel substrate is heated to about 435°C for about six hours while passing an about 88:12 (vol:vol) mixture of CO and hydrogen through the substrate at a total flow rate of about 500 cc/min. The carbon layer formed covers substantially all of the exposed surfaces of the substrate. The deposition of the precursor metal species on the carbon-coated metal substrate is accomplished by non-aqueous impregnation. Sufficient sodium is dissolved in anhydrous liquid ammonia to form a dark blue colored solution (about 0.6g sodium/lOOml liquid ammonia) . The liquid ammonia solution of sodium is forced through the carbon-coated metal substrate using a positive pressure of nitrogen gas. The ammonia is then removed by heating the membrane to about 200°C while purging with dry ultrapure nitrogen. The activation of the membrane to generate active sites which include sodium species in substantially deoxygenated form is then accomplished by heating the impregnated membrane for about 12 hours at a temperature of about 500°C while purging the membrane with a dry 35:65 (vol:vol) mixture of hydrogen gas and ultrapure nitrogen gas. Examples 2-6 Removal of Impurities from a Nitrogen Gas Stream The ability of reactive membranes prepared according to the procedure of Example 1 to remove impurities from a stream of inert gas is tested by passing a stream of ultrapure nitrogen gas, to which an impurity is added, through the -membrane. During the test the membrane is maintained at about room temperature while the nitrogen gas containing the impurity is passed through the membrane for the time period indicated at a flow rate of about 800-1000 cc/min. The input and output concentration of the impurity is measured at intervals throughout the duration of the test. The results are shown below in Table I. In all of the experiments shown, a steady state output impurity concentration is achieved very rapidly and maintained throughout the duration of the test. TABLE I Impurity Cone, (ppb) Time Input Output Ex. No. Impurity (mini Cone. Cone. 2 o 2 200 10,000 <0.1 3 H 2 0 200 1,020 0.6 4 CO 4600 350 <0.7 5 CH 4 200 100 <0.2 6 CO, 200 50 0.1 The results in Table I demonstrate that the reactive membranes of the present invention are highly effective for removing a variety of impurities from a gas. Example 7 Regeneration of a Reactive Membrane Including Sodium Species on Carbon-Coated Porous Alumina A Membralox * α-alumina coated with a monolayer of 7-alumina ceramic substrate available from Alcoa and having a pore size of about 50θA is degassed at about 400°C under a nitrogen atmosphere to remove any impurities in the substrate. Several monolayers of carbon are deposited on the ceramic substrate by the disproportionation of carbon monoxide using a gas mixture of 10 percent by volume CO, 2 percent by volume C0 2 and 88 percent by volume N 2 . The layers of carbon substantially entirely cover the surface area of the ceramic substrate to reduce the pore size thereof to about 5θA. Sodium hydroxide is vaporized at a temperature of about 300°C and the sodium gas is passed through the carbon coated ceramic substrate. This deposits (via CVD) a uniform layer of sodium metal in the oxidized form. An active group is formed between the carbon and sodium by heating the ceramic substrate layer in nitrogen gas to about 380°C to about 400°C. After heating, the sodium exists primarily in the C-Na form, i.e., as a substantially deoxygenated sodium species, which is highly reactive with many impurities. A nitrogen gas stream, which includes 2000 ppb water, is passed through the reactive membrane for 15 hours. The reactive membrane is then regenerated by heating the membrane for about 10 hours to from about 460°C to about 475°C while purging with a dry 35:65 mixture of hydrogen and ultrapure nitrogen gas. After regeneration the membrane shows water scavenging activity comparable to that achieved following the original activation process. In the specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims: THAT WHICH IS CLAIMED IS: 1. A reactive membrane for removing impurities from a gas comprising: (a) a porous metal substrate; and (b) at least one carbon layer deposited on the porous metal substrate, the carbon layer being modified to present active sites which include at least partially deoxygenated metal species chemically bonded to the carbon layer. 2. The reactive membrane according to claim 1 wherein the porous metal substrate includes stainless steel. 3. The reactive membrane according to claim 2 wherein the porous metal substrate includes sintered stainless steel powder or sintered stainless steel fibers. 4. The reactive membrane according to claim 2 wherein the stainless steel comprises 316L stainless steel. 5. A method for forming a reactive membrane for removing homogeneous and heterogeneous impurities from gases, said method comprising: (a) depositing at least one layer of carbon on a porous metal substrate; (b) depositing a precursor metal species on the carbon layer; and (c) heating the porous metal substrate to form active sites on the carbon layer, wherein the active sites include at least partially deoxygenated metal species chemically bonded to the carbon layer. 6. The method of claim 5 wherein depositing at least one layer of carbon includes depositing a carbon film on the porous metal substrate. 7. The method of claim 5 wherein heating the porous metal substrate includes heating the substrate in a reducing atmosphere. 8. The method of claim 7 wherein the reducing atmosphere is a hydrogen-containing atmosphere. 9. The method of claim 5 wherein depositing a metal precursor species includes contacting the carbon layer with a precursor metal species in a non-aqueous solvent. 10. The method of claim 9 wherein the non- aqueous solvent is anhydrous ammonia. 11. A method for forming a reactive membrane for removing homogeneous and heterogeneous impurities from gases, said method comprising: (a) depositing at least one layer of carbon on a porous substrate; (b) depositing a precursor metal species on the carbon layer by contacting the carbon layer with a precursor metal species in a non-aqueous solvent; and (c) heating the carbon layer to form active sites including at least partially deoxygenated metal species chemically bonded to the carbon layer. 12. The method of claim 11 wherein the non- aqueous solvent includes anhydrous ammonia. 13. The method of claim 11 wherein the precursor metal species include magnesium. 14. The method of claim 11 further comprising purging the carbon layer with an inert gas after depositing the precursor metal species. 15. The method of claim 14 further comprising heating the carbon layer to a temperature greater than 110°C after depositing the precursor metal species. 16. The method of claim 11 wherein depositing a precursor metal species includes depositing a precursor metal species slurried in a non-aqueous solvent. 17. The method of claim 11 wherein contacting the carbon layer with a precursor metal species in a non-aqueous solvent includes forcing the mixture containing the precursor metal species through the porous substrate with an inert gas. 18. The method of claim 11 wherein contacting the carbon layer with a precursor metal species in a non-aqueous solvent includes suctioning the mixture containing the precursor metal species through the porous substrate. 19. A method for activating a reactive membrane comprising heating a porous substrate which includes a carbon layer having active sites to greater than about 300°C in a reducing atmosphere. 20. The method of claim 19 wherein the reducing atmosphere is a hydrogen-containing atmosphere. 21. The method of claim 20 wherein the hydrogen-containing atmosphere includes greater than 1% hydrogen (by volume) . 22. The method of claim 19 wherein heating a porous substrate includes partially deoxygenating metal species to form active sites. 23. The method of claim 19 wherein heating the porous substrate includes binding the active sites to the carbon layer. 24. A method for processing a gas stream comprising: (a) establishing a flow of gas through a reactive membrane comprising a porous substrate and at least one carbon layer deposited on the porous substrate, the carbon layer being modified to present active sites which include metal species; (b) interrupting the gas flow through the reactive membrane; (c) heating the reactive membrane to greater than about 300°C in a reducing atmosphere, thereby regenerating the active sites; and (d) reestablishing the gas flow through the reactive membrane. 25. The method of claim 24 wherein the reducing atmosphere is a hydrogen-containing atmosphere. 26. The method of claim 25 wherein the hydrogen-containing atmosphere includes from about 2% to about 100% hydrogen (by volume) . 27. The method of claim 25 wherein heating the reactive membrane further comprises heating the reactive membrane in a reducing atmosphere for a period of time in the range from about 8 to about 24 hours.

Inventors: SHADMAN FARHANG F, SHADMAN, FARHANG, F.
Applicants: UNIV ARIZONA, THE UNIVERSITY OF ARIZONA
Application year: 1994
Main classification: B01J20
IPC codes: B01D39, B01D53, B01D69, B01D71, B01J12, B01J20